NT Seasonal Rainfall Outlook: probabilities for August to October 2004,
issued 13th July 2004

Three-month rainfall odds close to 50:50 in the NT

There are no strong shifts in the odds towards either above or
below average three-month falls for the August to October period,
the Bureau of Meteorology announced today.

The current pattern of outlook probabilities is due to mainly
near average temperatures in the Pacific and Indian Oceans.

For the Northern Territory, the odds of above median rainfall
are close to 50:50, meaning that they're too weak to offer any firm
guidance as to the most likely outcome for late dry to early wet
season rainfall. So in years with ocean patterns like the current,
about five August to October periods out of ten are expected to be
wetter than average, with about five out of ten being drier.

However, it should be noted that in much of the Territory, particularly
the Arnhem and Roper-McArthur districts, August to October is typically
a dry time of year and heavy rain is uncommon during this period.

Outlook confidence is related to the influence of Pacific and Indian
Ocean temperatures on seasonal rainfall. During August to October, history
shows this influence to be moderately consistent across the east and
north of the NT (see background information).

Both the Indian and tropical Pacific Oceans remain a little warmer
than average. For more detail see the the El Niño Wrap-Up.

The Southern Oscillation Index (SOI) has continued its strongly
fluctuating behaviour with a June value of 14, following
the May reading of +13 and April's 15.
The approximate SOI for the 30 days ending 10th July was 16.

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More information on this outlook is available from
9.00am to 5.00pm (CST) Monday to Friday by contacting
the Climate and Consultancy section in the Bureau's
Darwin Office: (08) 8920 3813.

THE NEXT ISSUE OF THE SEASONAL OUTLOOK IS EXPECTED BY 17th AUGUST 2004.

Probability outlooks should not be used as if they were categorical forecasts.
More on probabilities is contained in the booklet
The Seasonal Climate Outlook - What it is and how to use it,
available from the National Climate Centre.
These outlooks should be used as a tool in risk management
and decision making.
The benefits accrue from long-term use, say over 10 years.
At any given time, the probabilities may seem inaccurate,
but taken over several years, the advantages of taking account
of the risks should outweigh the disadvantages.
For more information on the use of probabilities,
farmers could contact their local departments of agriculture or primary industry.

Model Consistency and Outlook Confidence:
Strong consistency means that tests of the model on historical
data show a high correlation between the most likely
outlook category (above/below median)
and the verifying observation (above/below median). In this
situation relatively high confidence can be placed in the
outlook probabilities.
Low consistency means the historical relationship, and
therefore outlook confidence, is weak.
In the places and seasons where the outlooks are most skilful,
the category of the eventual outcome (above or below median)
is consistent with the category favoured in the outlook
about 75% of the time.
In the least skilful areas,
the outlooks perform no better than random chance or guessing.
The rainfall outlooks perform best in eastern and
northern Australia between July and January,
but are less useful in autumn and in the west of the continent.
The skill at predicting seasonal maximum temperature peaks in
early winter and drops off marginally during the second half of the year.
The lowest point in skill occurs in early autumn.
The skill at predicting seasonal minimum temperature peaks in
late autumn and again in mid-spring.
There are also two distinct periods when the skill is lowest
- namely late summer and mid-winter.
However, it must always be remembered that the outlooks
are statements of chance or risk.
For example, if you were told there was a 50:50 chance
of a horse winning a race but it ran second,
the original assessment of a 50:50 chance could still have been correct.

The Southern Oscillation Index (SOI) is calculated using the
barometric pressure difference between Tahiti and Darwin.
The SOI is one indicator of the stage of El Niño or La Niña
events in the tropical Pacific Ocean.
It is best considered in conjunction with sea-surface temperatures,
which form the basis of the outlooks.
A moderate to strongly negative SOI (persistently below 10) is
usually characteristic of El Niño, which is often associated with below
average rainfall over eastern Australia, and a weaker than normal monsoon
in the north. A moderate to strongly positive SOI (persistently above +10)
is usually characteristic of La Niña, which is often associated with
above average rainfall over parts of tropical and eastern Australia,
and an earlier than normal start to the northern monsoon season.
The Australian impacts of 23 El Niño events since 1900 are summarized
on the Bureau's web site
(www.bom.gov.au/climate/enso/).